The present invention pertains to rigid urethane-diol oligomer copolymers and the method of their preparation.
Graft copolymers are understood as the type of high-molecular-weight compounds, which have a main chain, i.e., backbone, chemically different from its side chains, i.e., branches, (Bruno Volmers, Polymer Chemistry (English translation) Springer Verlag, N.Y. 1973, pp. 288-300).
It is known that graft copolymers may be prepared by various methods which have a common fundamental feature, e.g., the basic polymer is always a starting material and the side chains are created on it either by polymerization of another monomer or by direct coupling. One of the often used methods of grafting by polymerization is the so called chain transfer where the basic polymer is dissolved in monomer, e.g., polyacrylate in styrene, and a suitable initiator is added which initiates the polymerization. In such case, the resulting composition or product (a mixture of graft copolymer and homopolymer) depends on the transfer constant. Because this constant is usually very low (about 10-5), the yield of copolymer is relatively low. However, such cases occur where the transfer constant is high due to the presence of relatively labile H-atoms in the vicinity of a double bond, e.g., with polybutadiene and its derivatives.
The polymerization of the main chain may be also started with X-rays, .gamma.-rays, etc., where a polymer radical is formed which is able to initiate the polymerization of the monomer present.
If the polymer molecule contains double bonds, it can be employed for the copolymerization of radical type, as it is, for example, with the gelation of unsaturated polyesters by copolymerization with styrene. Such tetrafunctional compounds (i.e. having two terminal ethylenically unsaturated groups) lead to a polymer having "a covalent chemical network" meaning a polymer which is covalently crosslinked, remaining always crosslinked and forever non-reversible.
Another known method is the introduction of functional groups into the basic polymer, e.g., peroxo, azo, CO.sub.2 H, OH, ##STR1## NCO, COCl, COOCO, or CH.sub.2 O groups, which then take part in the reaction either with a monomer or with an oligomer containing a suitable operating terminal group.
The present invention pertains to the formation of rigid urethane-diol oligomer grafted copolymers, wherein the basic polymer chain is formed by monomer units selected from the group comprising acrylates, methacrylates, acrylamide, styrene and N-vinyl-2-pyrrolidone, and the side branches are formed by 10-80 weight percent of oligourethanes based on aliphatic, aromatic and alkylaromatic diisocyanates and aliphatic low-molecule-weight diols containing 2-4 carbon atoms.
The method for preparation of the grafted copolymers according to the invention consists in the synthesis of oligourethane in the first step, which is copolymerized by radical mechanism with the low-molecular-weight vinyl monomers, selected from the group comprising acrylates, methacrylates, acrylamide, styrene, and N-vinyl-2 pyrrolidone, in the second step.
It has been found that the grafted copolymer, with the basic chain created by polymerization of vinyl monomer units and with oligourethanes as side branches, may be prepared in such a way, that an oligourethane based on aliphatiic, aromatic, and alkylaromatic diisocyanates and low-molecular-weight diols with 2-4 carbon atoms and a terminal unsaturated group, is synthesized in the first step, and then, in the second step, is subjected to the radical copolymerization with the low-molecular-weight monomer selected from the group comprising acrylates, methacrylates, acrylamide, styrene and N-vinyl-2-pyrrolidone. The terminal unsaturate double bond is introduced into the molecule of oligourethane in the first reaction step by the presence of 0.03 to 0.25 mol of hydroxyalkyl acrylate or hydroxyalkyl methacrylate, as 2-hydroxy-ethyl acrylate, 5-hydroxy-3-oxapentyl acrylate, 8-hydroxy-3,6-dioxaoctyl acrylate, 2-hydroxyethyl methacrylate, 5-hydroxy-3-oxapentyl methacrylate, and 8-hydroxy-3,6-dioxaoctyl methacrylate, calculated on 1 mol of diisocyanate used.
As follows from published data, the reaction of 2-hydroxy-ethyl methacrylate with dissocyanate led, however, to the corresponding diurethane, i.e. from the standpoint of radical polymerization, to the tetrafunctional monomer which acted as a crosslinking agent in copolymerization (S. S. Labana: J. Polymer Sci., A-1 6, 3283 (1968). The synthesis of tetrafunctional compound is similarly described by Yu. L. Spirin, V. V. Magdinets, A. F. Maslyuk, L. N. Vilenskaya and A. R. Rudko in the Soviet Union Pat. No. 273,424 (1970).
We have found that the formation of diurethane depends on the molar ratio of hydroxyalkyl acrylate or methacrylate to diisocyanate and that the limit of the molar ratio, below which diurethane is formed only in traces in addition to monourethane, is 0.25 mol to 1 mol of diisocyanate. A suitable aliphatic diisocyanate is hexamethylenediisocyanate, suitable aromatic diisocyanates are p-phenylenediisocyanates, 2,4-tolylenediisocyanate, 2,6-tolylenediisocyanate (or their mixture), m-xylylenediisocyanate, 1,5-naphthalenediisocyanate, suitable alkylaromatic diisocyanates are 4,4'-methylenebis-(phenylisocyanate) and 4,4'-ethylenebis(phenylisocyanate). The bifunctional oligourethane is formed by polyaddition reaction in the presence of low-molecular-weight diols, as ethylene glycol, propylene glycol, 1,4-butanediol, and 3-oxa-1,5-pentandiol.
Oligourethane having a vinyl group may be prepared stepwise, i.e. diisocyanate is allowed to react with hydroxy-alkyl acrylate or methacrylate first and on completion of that reaction with a corresponding diol. The reaction may be also carried out in one step, when both hydroxy compounds, the monohydroxy and dihydroxy, react with diisocyanate at the same time. Both in the first and in the second case, hydroxyalkyl ester acts as a terminator and, consequently, the molecular weight of the resulting oligourethane depends on the molar ratio of hydroxy ester to diisocyanate (in addition to the degree of conversion), as illustrates Table I.
______________________________________ Example no. mol HEMA --M.sub.n x.sub.E ______________________________________ 1 0.25 1160 20.7 2 0.20 1560 15.3 3 0.15 2040 11.9 4 0.10 2280 10.7 5 0.05 3600 6.9 ______________________________________ x.sub.E mol % of double bonds in oligourethane.
The product may be isolated, on completion of the reaction, by precipitation into water, filtration and drying to constant weight and used for copolymerization. The reaction product can be used without isolation, if the concentration of OH-groups was chosen higher than an equivalent, with respect to the concentration of NCO-groups. In such case, oligourethane is terminated with the hydroxyl groups as shows the formula of assumed structure: ##STR2## where
R belongs to the used diol
R' belongs to the used diisocyanate
X is H or CH.sub.3.
It has been proven, that oligourethane prepared in this way is able to copolymerize with low-molecular-weight vinyl monomers selected from the group comprising acrylates, methacrylates, acrylamide, styrene and N-vinyl-2-pyrrolidone, thus forming the grafted copolymers which have the basic chain formed by constitutional structural units of the above monomers and the branches formed from oligourethanes. These copolymers are meltable and soluble under certain conditions. The conditions are given by both the chemical structure of main chain and the chemical structure of the branches.
As will be seen from the foregoing, the grafted polymer produced in accord with the present invention has a structure providing crystallinic rigid microareas serving as a physical network which enables the transition of the material into a non-crosslinked composition by simple heat treatment.
According to the present invention when the ratio of the hydroxyalkyl acrylate or methacrylate to the oligourethane is held in the range of 0.03 to 0.25 mol of hydroxyalkyl acrylate or methacrylate to 1 mol of diisocyanate, compounds with only one terminal ethylenic unsaturation are formed. When copolymerizing the compounds of the invention, copolymers are formed which have only a physical network, contrary to the chemically covalently cross-linked copolymers discussed earlier. The compounds of the present invention are reversible due to the microheterogeneous structure which is formed in consequence to the fact that the individual components are mutually immiscible. This physical network can be destroyed by heating the system to a temperature above the melting point of the polyurethane component and can be formed again when cooling the system.
The resulting copolymers combine the physical and mechanical properties of their components, similarly as it is in block copolymers of the ABA or (AB) n type (so called segmented copolymers), according to the content of individual components.
These properties enable a broad application of graft copolymers according to the invention. The combination of a hydrophilic and lipophilic components leads, for example, to a product having the character of nonionogenic emulsifiers for the system water--organic phase; connection of a component with the tendency to crystallization with the basic elastomeric chain gives products of enhanced elasticity modulus.
The advantage of preparation of graft copolymers according to the invention consists in the easy method of preparation on the one hand and in connection of entirely different types of macromolecular compounds on the other.
According to our invention the synthesis must be carried out in the presence of an aprotic solvent such as dimethylformamide (DMF) or dimethylsulfoxide (DMS) from which the oligomer is isolated per se and then reacted with the appropriate monomer such as acrylate, methacrylate, acrylamide, styrene, N-vinylpyrrolidon.
The reaction takes place in the following way. First the diisocyanate is reacted with HEMA and the formed product with the diol thus forming the oligomer having only one terminal ethylenically unsaturated group, which is afterwards copolymerized with the above monomers. Due to the presence of only one unsaturated group there is formed a copolymer which is not cross-linked but has only a physical network which is dry, rigid, highly disposed to crystallization, and not elastic, but which is reversible due to the microheterogeneous structure which is formed in consequence to the fact that the crystallinic phase and the amorphous phase are mutually immiscible, which network can be in contradistinction to the covalent crosslinking, destroyed by heating and renewed by cooling the system.
The invention is further illustrated in several examples of performance wihout, however, limiting its scope by any means.